2.1 Primary and Secondary Triads

Diatonic harmony is all about chords built from the notes in a key. Primary triads (tonic, subdominant, dominant) are the big players, while secondary triads (supertonic, mediant, submediant) add flavor. These chords form the backbone of tonal music.

Understanding triad qualities and inversions is key to analyzing chord progressions. Roman numeral analysis helps identify chord functions within a key. Mastering these concepts opens up a world of harmonic possibilities in composition and analysis.

Primary Triads

Tonic, Subdominant, and Dominant Triads

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• Primary triads are the most important chords in a key and include the tonic, subdominant, and dominant triads
• The tonic triad is built on the first scale degree and represents the key center or tonal center of a piece (C major in the key of C)
• The subdominant triad is built on the fourth scale degree and creates a sense of departure from the tonic (F major in the key of C)
• The dominant triad is built on the fifth scale degree and creates a strong pull back to the tonic (G major in the key of C)

Major and Minor Triad Qualities

• Primary triads can be either major or minor, depending on the quality of the key
• In a major key, the tonic and subdominant triads are major, while the dominant triad is minor (C major, F major, G minor in C major)
• In a minor key, the tonic and subdominant triads are minor, while the dominant triad is major (A minor, D minor, E major in A minor)
• A major triad consists of a root, major third, and perfect fifth (C, E, G in C major)
• A minor triad consists of a root, minor third, and perfect fifth (A, C, E in A minor)

Secondary Triads

Supertonic, Mediant, and Submediant Triads

• Secondary triads are less important than primary triads and include the supertonic, mediant, and submediant triads
• The supertonic triad is built on the second scale degree (D minor in the key of C)
• The mediant triad is built on the third scale degree (E minor in the key of C)
• The submediant triad is built on the sixth scale degree (A minor in the key of C)

Diminished Triad on the Leading Tone

• In addition to the secondary triads, there is a diminished triad built on the seventh scale degree, also known as the leading tone
• A diminished triad consists of a root, minor third, and diminished fifth (B, D, F in the key of C)
• The diminished triad on the leading tone creates a strong pull towards the tonic, as the leading tone naturally resolves to the tonic
• In a minor key, the leading tone triad is diminished, while in a major key, it is a diminished triad borrowed from the parallel minor key (B diminished in C major, borrowed from C minor)

Triad Analysis

Roman Numeral Analysis and Diatonic Triads

• Roman numeral analysis is a system for labeling chords based on their scale degree and quality
• Diatonic triads are triads built on the notes of a particular key, using only the notes belonging to that key
• In major keys, the diatonic triads are: I (major), ii (minor), iii (minor), IV (major), V (major), vi (minor), and vii° (diminished)
• In minor keys, the diatonic triads are: i (minor), ii° (diminished), III (major), iv (minor), v (minor), VI (major), and VII (major)

Triad Qualities and Inversions

• Triad qualities refer to the intervals that make up the triad, such as major (M3 + m3), minor (m3 + M3), diminished (m3 + m3), and augmented (M3 + M3)
• Triad inversions occur when the lowest note of the triad is not the root
• First inversion triads have the third as the lowest note and are labeled with a "6" (C/E)
• Second inversion triads have the fifth as the lowest note and are labeled with a "64" (C/G)
• Inversions are indicated in the roman numeral analysis by adding the inversion symbol after the roman numeral (I6, ii64)